Graphene is a two-dimensional crystal of a hexagonal honeycomb-like structure consisting of a single layer of carbon atoms. An ideal graphene structure is a planar hexagonal lattice, which can be regarded as a stripped single layer of a graphite molecule with each carbon atom being sp2 hybridized and contributing the remaining p-orbital electron to the formation of a delocalized π bond where π-electrons are freely movable, imparting the graphene with good conductivity. The two-dimensional graphene structure can be seen as the basic structural element of all sp2 hybridized carbon materials. Graphene contains approximately 0.143 nm long C—C bonds as well as lattice elements each having three σ bonds which are rather strong and ensure stability of the hexagons. A single graphene layer has a thickness of only 0.35 nm, about 1/200,000 of the diameter of a hair shaft.
Graphene has been found to have many novel and unique physical phenomena to date such as massless Dirac fermions, the quantum Hall effect, room-temperature magnetic behavior and room-temperature spin-dependent transport. More importantly, graphene with the unique structure has multiple excellent properties, including a small mass density (2.2 g/cm3), extremely large specific surface area (˜2630 m2/g), extremely high intrinsic mobility (>104 cm2/Vs, higher than ten times that of commercial silicon wafers), resistivity of only about 10−6 Ω·cm, the lowest as so far found in the world, outstanding thermal conductivity (3000 W·m−1·K−1, 3 times that of diamond) and mechanical characteristics (its Young's modulus is as high as up to 1.0 TPa, higher than 100 times that of steel). In graphene, electrons move faster than in pure metals and semiconductors at a speed that is up to 1/300 of the speed of light. Although graphite is the softest mineral whose Mohs hardness is in the range of only 1 to 2, after being exfoliated into graphene with a thickness equal to the diameter of a single carbon atom, it will undergo abrupt changes in properties and exhibit a Mohs hardness higher than that of diamond (10) and high strength of up to 130 GPa, while maintaining high toughness and arbitrary flexibility. In addition, graphene is nearly transparent with an optical absorbance of only 2.3% over a wide wavelength band. Thanks to its fastest room-temperature electrical conduction velocity, greatest mechanical strength, strongest thermal conductivity and other properties, it is potentially applicable to critical applications including nanoelectronic devices, optoelectronic devices, sensors, super capacitors, energy storage, integrated circuits for fuel cells, transistors and FETs.
Graphene/polymer composite materials can be prepared from in-situ growth of organic molecules introduced between graphite layers through melt blending, solution blending, solution intercalation or the like. In addition to its wide variety of excellent properties including high strength, high conductivity, high light transmittance, high hardness, high bather ability and quantum Hall effect at room temperature, graphene is suitable for use in many applications also because of the abundance and low cost of its raw materials (graphite, hydrocarbons, etc.). Preparation through addition of graphene to polymers is currently the focus of research on the development of high-performance, multi-functional, intelligent composite materials. However, direct addition of graphene to polymers tends to fail to meet the requirements of actual production, as further functional modifications and structural designs are necessary for enabling their functionality and intelligence.
Among the multi-functional, intelligent composite materials, the preparation of piezo-resitive-responsive composites usually employs techniques using carbon black, carbon nanotubes, or regular graphene as filler. These techniques, however, are associated with a number of issues. For example, piezoresistive composites containing carbon black as filler suffer from low sensitivity, high filler contents and unsatisfactory repeatability. While those prepared using carbon nanotubes as filler exhibit relatively high piezoresistive sensitivity and low filler contents, such excellent piezoresistive characteristics cannot be repeated as expected in cycle tests. There have also been attempts using graphene as filler in which alkylation is further performed in order to enhance the bonding between the conductive filler and polymer matrix and hence the piezoresistive response. However, this only mitigates the interfacial bonding issue but fails to address the interfacial bonding and piezoresistive response issues fundamentally from the root causes. For these reasons, we developed a graphene/conductive polymer composite material by sulfonation of graphene surface by grafting and subsequent bonding of in-situ polymerized poly(3,4-ethylenedioxythiophene) (PEDOT) using hydrogen bonds. This material can be used alone as, or composited with polymer(s) to form, a piezo resistive material.
Research efforts made in sofar in multi-functional, intelligent graphene/polymer composite materials are essentially as follows: Chinese Patent Publication No. CN102173145A of Chinese Patent Application No. 201210144498.X, filed May 10, 2012, published Sep. 19, 2012, entitled “Surface Modified Graphene/Polymer-Based Piezoresistive Composite Material and Preparation Method thereof”, which discloses a piezoresistive-responsive material prepared by compositing silicone rubber with alkylated graphene as filler.
Chinese Patent No. CN102558772B of Chinese Patent Application No. 201110419526.X, filed Dec. 15, 2011, issued Mar. 6, 2013, entitled “PEDOT/Sulfonated Graphene Composite Hydrogel and Preparation Method Thereof”, which discloses a composite hydrogel with improved electrical properties and mechanical strength formed by mixing PEDOT, sodium polystyrene sulfonate and sulfonated graphene as the individual constituents.
Chinese Patent Publication No. CN103824615A of Chinese Patent Application No. 201410055227.6, filed Feb. 18, 2014, published May 28, 2014, entitled “Method for Flexible Transparent Electrode of Laminated Vapor-Phase Polymerized PEDOT and Graphene”, which discloses a composite sheet with improved electrical conductivity and flexibility prepared by laminating PEDOT and graphene films.
These research efforts, however, fail to address the interfacial bonding issue while achieving enhancements in electrical conductivity, mechanical strength, flexibility, etc.